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Northern Rockies Skies for June

May 30, 2012 — A monthly look at the night skies of the northern Rocky
Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay
Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.

Although the June night skies may appear empty compared to
the variety and diversity of the winter and spring constellations and planets,
they do present a spectacular display of interesting objects.

To get oriented: Overhead, you will see an unobstructed view
of the Big Dipper and, on the northern horizon the notable constellation
Cassiopeia, now a giant stretched-out "W" in the sky. Find Polaris, the North
Star, by using the pointer stars -- the two stars on the outside edge of the Big Dipper's cup. Here is a good chance to "arc to Arcturus." Follow the curved
handle of the Big Dipper and you reach Arcturus, and then Spica in Virgo.

On the eastern horizon, you will first see rising the bright
star Vega, then Deneb (more northerly) and finally Altair which, in combination,
make up the summer triangle.

Going toward the southern horizon, you will notice the curlicue-shaped
constellation Scorpios with its bright, red star, Antares. The Milky Way runs
almost parallel to the southeast horizon through these constellations. You are
now looking very close to the center of the Milky Way galaxy.

At sunset, you can see Saturn near Spica in Virgo and Mars
midway between Spica and Regulus in Leo the Lion. A partial lunar eclipse is
visible June 4 and the transit of Venus across the sun can be seen June 5.

The apparent diameter of the sun is sufficiently large
(1/2 degree) and the orbital planes of Earth and our sister planet Venus
sufficiently similar, that Venus infrequently transits, or crosses in front of
the sun, as seen from Earth. These transits occur in pairs separated by eight
years, with the pairs currently recurring after gaps of 105.5 or 121.5 years --
a "twice in a human lifetime event" if one happens to be born in a lucky
year. The last transit occurred June 8, 2004, and its paired event will occur June
5-6.

In the United States, the start of the transit will be
visible June 5, weather permitting, just after 4 p.m. MDT, with time of
greatest excursion onto the sun's disk around 7:25 p.m. During the transit the
apparent diameter of Venus will be close to one arc minute, or about 1/30 of
the sun's diameter.

A small telescope (mirror or objective lens just a few
inches in diameter) combined with an eyepiece providing low magnification
(about 10-40 times) are quite ample to study progress of the black disk of
Venus against the bright sun.

However, maximum warning -- the most important and
imperative piece of equipment -- is a solar filter to block 99.999 percent of the sun's
light. Without this filter, just an instant of looking at the sun through even
a small telescope (or binoculars) will result in severe and often permanent
damage to the eye's retina (this happened to the author while viewing the sun
emerging from total eclipse).

Alternatively, but using a similar solar filter (e.g.,
grade 14 welder's glass), the transit can be viewed without aid of a telescope,
as Venus' apparent diameter will be comparable to large sunspots.

Historically, transits of Venus were employed to obtain
an accurate measure of the distance between Earth and the sun through the
method of parallax. The next pair of transits of Venus will take place in the
next century, in 2117 and 2125.